For current communication technology, telecommunication or optical fiber communication has been widely used.
In the communication method that uses electricity or light, the signal transmission speed is limited below speed of light.
On the other hand, the quantum communication technology or quantum cryptography based on the principle of quantum mechanics is being actively researched.
Moreover, the quantum teleportation, reproducing a quantum state in another system, is also being researched using the entangled state.
In the quantum code or the quantum teleportation, the collapse of the wave packet (decoherence) is used. The collapse of the wave packet occurs instantly when measurement is done, and a strong correlation appears in each measurement result of each part in the entangled state.
However, it is said that it is not possible to use the entangled state to send information because an individual measurement result of the entangled state is quite random, and cannot arbitrarily choose the measurement result.
Therefore, even in the quantum code or the quantum teleportation, the communication process at the speed below speed of light is needed to actually send information. So the signal transmission speed becomes below speed of light.
Related references are as follows:
Japanese Patent Publication No. 2010-252831
Phys. Rev. Lett. 75, 4337-4341(1995)
The quantum theory of light, second edition, Oxford University Press 1973, 1983